Coated grooving or parting insert and method of making same

ABSTRACT

The present invention relates to a coated cutting tool (e.g.—cemented carbide insert) useful for grooving or parting of steel or stainless steel components such as tubes and bars. The insert is characterised by a WC-Co-based cemented carbide substrate having a highly W-alloyed Co-binder phase and a hard and wear resistant coating including a multilayered structure of sublayers of the composition (Ti x Al 1-x )N with a periodic and repeated variation of the Ti/Al ratio.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a reissue of U.S. Pat. No. 6,342,291 B1,which claims the benefit of priority of Swedish Application No.9903089 - 2 filed Sep. 1, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a coated cutting tool (cemented carbideinsert) useful for grooving or parting of steel components, such as barsor tubes, not only of stainless steels of different compositions andmicrostructures but also of non-stainless steels such as low carbonsteels and low and medium alloyed steels.

2. Description of Related Art

In the discussion of the state of the art that follows, reference ismade to certain structures and/or methods. However, the followingreferences should not be construed as an admission that these structuresand/or methods constitute prior art. Applicant expressly reserves theright to demonstrate that such structures and/or methods do not qualifyas prior art against the present invention.

When machining low and medium alloyed steels and stainless steels withcemented carbide tools, the cutting edge is worn according to differentwear mechanisms. Examples of such mechanisms include chemical wear,abrasive wear, adhesive wear and edge chipping. In bad cuttingconditions problems with bulk and edge line breakages commonly occur.Furthermore, different cutting variables, such as cutting speed andcutting feed rate, and external conditions, such as dry or wet machiningand heavy vibrations of the work piece, require a plurality of differentproperties of the cutting edge.

It has been very difficult to improve all of the plurality of preferredtool properties simultaneously. Therefore, commercial cemented carbidegrades have been optimized with respect to one or only a few of thesewear types and, hence, to specific application areas. U.S. Pat. No.6,062,776 discloses a coated cutting insert particularly useful for dryand wet machining in low and medium alloyed steels and stainless steels,with or without raw surface zones, under severe conditions such asvibration, long overhand, and recutting of chips. The insert ischaracterized by a WC-Co cemented carbide with a low content of cubiccarbides, a rather low W-alloyed binder phase, and a coating includingan innermost layer of TiC_(x)N_(Y)O_(z) with columnar grains and a toplayer of TiN and an inner layer of κ-Al₂O₃.

Swedish patent application SE 9901149-6 discloses a coated cuttinginsert, particularly useful at high cutting speeds, for use on stainlesssteels of different compositions and microstructures. The insert is alsouseful for the milling of non-stainless steels such as low carbon steelsand low and medium alloyed steels. The coated WC-Co based cementedcarbide insert is characterized by a specific composition range of WC/Cowithout any addition of cubic carbides, a low W-alloyed Co binder, anarrowly defined range of average WC grainsizes, and a hard and wearresistant coating including a multilayered structure of sublayers of thecomposition (Ti_(x)Al_(1-x))N with repeated variation of the Ti/Alratio.

SUMMARY OF THE INVENTION

It has now surprisingly been found that a combination of a modifiedcemented carbide substrate described in the above mentioned U.S. Pat.No. 6,062,776 and the coating described in the above mentioned SE9901149-6 results in excellent cutting performance in grooving or, inparticular, parting of steel, including stainless steel.

A cutting tool insert, for parting of steel and stainless steel, and amethod of making the same is provided. The cutting tool insert iscomprised of a cemented carbide body and a coating wherein the cementedcarbide body consists of WC with an average grain size of about 1.4 mm,12-13 wt-% Co, 0.4-1.8 wt-% TaC+NbC, and a low W-alloyed binder phasewith a CW-ratio of 0.82-0.91. The coating is comprised of a multiplelayers. The first (innermost) layer is 0.1-0.5 μm of TiN. A second layeris comprised of a multilayered structure of 0.05-0.2 μm thick sublayers.The sublayers are of a composition (Ti_(x)Al_(1-x))N in which x variesrepeatedly between the two ranges 0.45<×<0.55 and 0.70<×<0.80. A firstsublayer, adjacent to the TiN bonding layer, is of a composition(Ti_(x)Al_(1-x))N having an x-value of 0.45<×<0.55. A second sublayer isof a composition (Ti_(x)Al_(1-x))N having an x-value of 0.70<×<0.80 anda third sublayer is of a composition (Ti_(x)Al_(1-x))N having x of0.45<×<0.55. The sequence of sublayers alternates and repeats until12-25 sublayers are built up. After the multilayered structure, a thirdlayer of (Ti_(x)Al_(1-x))N, where x is 0.45<×<0.55, is deposited to athickness of 0.1-0.5 μm. A fourth and outermost layer of TiN isdeposited to a thickness of 0.1-0.2 μm. The total coating thickness is1-8 mm and the thickness of the second layer constitutes 75-95% of thetotal coating thickness.

BRIEF DESCRIPTION OF THE DRAWING FIGURE

The objects and advantages of the invention will become apparent fromthe following detailed description of preferred embodiments thereof inconnection with the accompanying drawings in which like numeralsdesignate like elements and in which:

FIG. 1 is a micrograph of a polished cross section of a coated insertaccording to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a micorgraph at 1200×magnification of a polished crosssection of a coated insert according to the present invention in whichthe layers of the cross section are visible. The layers are a cementedcarbide body A, an innermost TiN layer B, a layer of several TiAlNsublayers C, a layer of TiAlN D, and an outermost TiN layer E.

According to the present invention, there is provided a coated cuttingtool insert that satisfies the toughness demands of machining operationssuch as the grooving and parting of stainless steels. The coated cuttingtool insert comprises a WC-Co based cemented carbide body with acomposition of 11.5-13.6 wt % Co, preferably 12.0-13.0 wt % Co, mostpreferably 12.3-12.9wt % Co, 0.2-1.8 wt % cubic carbides, preferably0.4-1.8 wt % cubic carbides, most preferably 0.5-1.7 wt % cubic carbidesof the metals Ta, Nb and Ti and balance WC. The cemented carbide mayalso contain other carbides from elements from group IVb, Vb or VIb ofthe periodic table. The content of Ti is preferably on a levelcorresponding to a technical impurity. The average grain size of the WCis about 1.1-2.1 μm, preferably about 1.4 μm.

The cobalt binder phase is rather low and alloyed with W. The content ofW in the binder phase can be expressed as the CW-ratio:CW−ratio=M_(s)/(wt % Co·0.0161)  (1)where M_(s) is the saturation magnetization of the cemented carbide bodyin kA/m hAm² /kg and wt % Co is the weight percentage of Co in thecemented carbide. The CW-value is a function of the W content in the Cobinder phase. A high CW-value corresponds to a low W-content in thebinder phase.

It has now been found according to the present invention that improvedcutting performance is achieved if the cemented carbide body has aCW-ratio of 0.80-0.92, preferably 0.82-0.91, and most preferably0.85-0.90. The cemented carbide may contain small amounts, <1 volume %,of η-phase (M₆C), without any detrimental effect. Form the CW-value itfollows that no free graphite is allowed in the cemented carbide bodyaccording to the present invention.

The hard and wear resistant refractory coating deposited on the cementedcarbide substrate A according to the present invention comprises a first(innermost) thin 0.1-0.5 μm bonding layer of TiN B.

The coating includes a second layer comprising a multilayered structureof sublayers C of the composition (Ti_(x)Al_(1-x))N in which x variesrepeatedly between the two ranges 0.45<×<0.55 and 0.70<×<0.80. The firstsublayer of (Ti_(x)Al_(1-x))N adjacent to the TiN bonding layer havingan x-value of 0.45<×<0.55, the second sublayer of (Ti_(x)Al_(1-x))Nhaving an x-value of 0.70<×<0.80 and the third sublayer having x of0.45<×<0.55 and so forth repeated until 12-25 sublayers, preferably22-24 sublayers, are built up. The thickness of this second layercomprising a multilayered structure of sublayers constitutes 75-95% ofthe total coating thickness. The individual sublayers of(Ti_(x)Al_(1-x))N are essentially of the same thickness but theirthickness may also vary in a regular or irregular way and said sublayerthickness is 0.05-0.2 μm.

The coating further includes a third 0.1-0.5 μm layer of(Ti_(x)Al_(1-x))N D having an x-value of 0.45<×<0.55, and a fourth(outermost) thin 0.1-0.2 μm layer of TiN E.

The total thickness of the coating is 1-8 μm, preferably 2-5 μm. Thelayer thickness, the sublayer thickness and the coating thickness quotedabove refer to measurements made close to the cutting edge, i.e.—thefunctional part of the cutting tool.

The present invention will now be further described by reference to thefollowing examples, which are intended to be illustrative ratherrestrictive.

EXAMPLE 1

A. A cemented carbide parting tool insert in accordance with the presentinvention with a composition of 12.6 wt-% Co, 1.25 wt-% TaC, 0.30 wt-%NbC and balance WC with a 1.4 μm grain size and with a binder phasealloyed with W corresponding to a CW-ratio of 0.91 was coated with a 4μm thick coating by applying a conventional PVD cathodic arc technique.The coating consisted of a first (innermost) 0.2 μm layer of TiNfollowed by a 3.2 μm thick second layer comprising 23 alternatingsublayers of (Ti_(x)Al_(1-x))N, where x alternatively varied between0.55 and 0.75, a third layer 0.2 μm (Ti_(x)Al_(1-x))N, where x=0.55,and, finally, an outermost 0.4 μm layer of TiN.

B. A cemented carbide parting tool insert not of the present inventionwith a composition of 8.0 wt-% Co, no cubic carbides, balance WC and aCW-ratio of 0.94. The insert was coated with an innermost 0.5 μmequiaxed TiCN-layer. A 1.5 μm TiN layer was deposited, during the samecycle, on top of the TiCN-layer. No post treatment was applied.

C. For comparison, a competitive cemented carbide parting tool insert instyle similar to previously mentioned inserts was obtained from anexternal leading cemented carbide producer. The carbide had acomposition of 12.5 wt-% Co, 0.1 wt-% TiC, 1.8 wt-% TaC, 0.2 wt-% NbC,and balance WC with a CW-ratio of 0.87. The insert had a coatingconsisting of 1.4 μm TiN and, outermost, 1.4 μm TiCN. Examination undera light optical microscope revealed no edge treatment subsequent tocoating.

Inserts A, B and C were tested in a parting off to center in stainlesssteel SS2321 with OD 26 mm. The cutting speed was varied from 86 to 0m/min with a feed rate of 0.05 mm/r. The wear mechanism was uneven flankwear and chipping.

Insert Number of components A 50 B 13 C 41

EXAMPLE 2

Inserts A and B were tested at an end user's machine shop in a partingof a stainless steel component (AISI 316 OD 42 mm). The cutting speedwas varied from 110 to 0 m/min with a feed rate varying from 0.08 to 003mm/r. The low feed rate was used close to the center of the bar. Thewear mechanism was fracture in the cutting zone.

Insert Number of components A 201 B 224

EXAMPLE 3

Inserts A and B were tested at an end user's machine shop in a partingof a steel component (SS2172 OD 47 mm). The rotating speed was 1800 rpmwith a feed rate of 0.1 mm/r. The wear mechanism was flank wear andflaking.

Insert Number of components A 163 B 50

EXAMPLE 4

Inserts A and C were tested at an end user's machine shop in a partingof a stainless steel component (AISI 316 OD 31 mm). The cutting speedwas varied from 60 to 0 m/min with a feed rate varying from 0.06 to 0.03mm/r. The low feed rate was used close to the center of the bar. Thewear mechanism was flank wear and chipping.

Insert Number of components A 182 B 43

The present invention also relates to a method of making a coatedcutting tool insert consisting of a cemented carbide body with acomposition of 11.5-13.6 wt % Co, preferably 12.0-13.0 wt % Co, mostpreferably 12.3-12.9 wt % Co, 0.2-1.8 wt % cubic carbides, preferably0.4-1.8 wt % cubic carbides, most preferably 0.5-1.7 wt % cubic carbidesof the metals Ta, Nb and Ti and balance WC. The cemented carbide mayalso contain other carbide from elements from group IVb, Vb or VIb ofthe periodic table. The content of Ti is preferably on a levelcorresponding to a technical impurity. The average grain size of the WCis about 1.1-2.1 μm, preferably about 1.4 μm.

The hard and wear resistant refractory coating is deposited onto thecemented carbide substrate by applying conventional PVD (Physical VaporDeposition) or CVD (Chemical Vapor Deposition) methods and, according tothe present invention, said coating comprises a first (innermost) thin0.1-0.5 μm bonding layer of TiN, a second layer comprising amultilayered structure of sublayers of the composition (Ti_(x)Al_(1-x))Nin which x varies repeatedly between the two ranges 0.45<×<0.55 and0.70<×<0.80. The first sublayer of (Ti_(x)Al_(1-x))N adjacent to the TiNbonding layer having an x-value of 0.45<×<0.55, the second sublayer of(Ti_(x)Al_(1-x))N having an x-value of 0.70<×<0.80 and the thirdsublayer having x of 0.45<×<0.55 and so forth repeated until 12-25sublayers, preferably 22-24 sublayers, are built up. The thickness ofthis second layer comprising a multilayered structure of sublayersconstitutes 75-95% of the total coating thickness. The individualsublayers of (Ti_(x)Al_(1-x))N are essentially of the same thickness buttheir thickness may also vary in a regular or irregular way and saidsublayer thickness is 0.05-0.2 μm, a third thin 0.1-0.5 μm layer of(Ti_(x)Al_(1-x))N having an x-value of 0.45<×<0.55 and a fourth(outermost) 0.1-0.2 μm layer of TiN.

Although the present invention has been described in connection withpreferred embodiments thereof, it will be appreciated by those skilledin the art that additions, deletions, modifications, and substitutionsnot specifically described may be made without department from thespirit and scope of the invention as defined in the appended claims.

1. A cutting tool comprising: a cemented carbide body comprising WC withan average grain size of about 1.4 μm, 12-13 wt-% Co, 0.4-1.8 wt-%TaC+NbC, and a low W-alloyed binder phase with a CW-ratio of 0.82-0.91;and a coating comprising: a first innermost 0.1-0.5 μm thick layer ofTiN; a second layer comprising a multilayered structure of 0.05-0.2 μmthick sublayers of a composition (Ti_(x)Al_(1-x))N in which x variesrepeatedly between the two ranges 0.45<×<0.55 and 0.70<×<0.80, a firstsublayer of (Ti_(x)Al_(1-x))N adjacent to the TiN bonding layer havingan x-value of 0.45<×<0.55, a second sublayer of (Ti_(x)Al_(1-x))N havingan x-value of 0.70<×<0.80 and a third sublayer having x of 0.45<×<0.55,the sequence of sublayers alternating and repeating until 12-25sublayers are built up; a third 0.1-0.5 μm thick layer of(Ti_(x)Al_(1-x))N, where x is 0.45<×<0.55; and a fourth outermost0.1-0.2 μm layer of TiN; wherein the total coating thickness is 1-8 μmand the thickness of the second layer constitutes 75-95% of the totalcoating thickness.
 2. The cutting tool according to claim 1 wherein thecemented carbide body has a composition comprising 12.3-12.9 wt-% Co and0.5-1.7 wt % TaC+NbC.
 3. The cutting tool according to claim 1 whereinthe cemented carbide body is free from graphite.
 4. The cutting toolaccording to claim 1 wherein the CW-ratio is 0.85 - 0.90.
 5. The cuttingtool according to claim 1 wherein the multilayered structure of thesecond layer has 22- 24 sublayers.
 6. The cutting tool according toclaim 1 wherein the individual sublayers of (Ti _(x) Al _(1-x))N areessentially of the same thickness.
 7. The cutting tool according toclaim 1 wherein the total coating thickness is 2- 5 μm.